1. Field of the Invention
The invention relates to an electric load drive circuit that drives a load installed in a vehicle. More particularly, the invention relates to an apparatus that monitors an abnormality of a sensor in the load drive circuit.
2. Description of the Related Art
Recently, a 42-volt electric power source system for an automobile, in which a 42-volt electric power source is added to a conventional 14-volt electric power source has been proposed, in order to supply electric power to electric equipment having a large capacity installed in the automobile, such as an electric power steering and an electric air conditioner. In the 42-volt electric power source, since the voltage is three times as high as the voltage in the conventional 14-volt electric power source, electric current can be reduced to one-third, which leads to a reduction in loss and a reduction in weight of the harness. Also, electric power can be supplied to a load having a large capacity more easily. The capacity of a battery is larger than a conventional battery. Further, there is a possibility that a nickel-hydrogen battery or a lithium ion battery will be employed instead of a lead battery that is currently employed.
However, the lifetime of a lamp and the like may be shortened when only the 42-volt electric power source is used. Therefore, the conventional 14-volt electric power source needs to be used as it is, together with the 42-volt electric power source. Thus, recently, an electric power source system including a 42-volt main battery, a 14-volt auxiliary battery, and a DC-DC converter for increasing and decreasing the voltage has been proposed. In the 14-volt battery, the charging voltage is 14 volts and the discharging voltage is 12 volts. In the 42-volt battery, the charging voltage is 42 volts and the discharging voltage is 36 volts.
In addition to such an electric power source system, there are cases where a low voltage of electric power discharged from a battery is increased by a DC-DC converter so as to supply the electric power to an inverter of a motor in a hybrid vehicle in which driving power of an engine is assisted by the motor. In such cases, a rated voltage of the motor for driving the vehicle is usually set to a high value in order to reduce weight of the harness by reducing electric current supplied to the motor for driving the vehicle, and to obtain high driving power for driving the vehicle. Meanwhile, in order to increase the voltage of the battery installed in the vehicle, a large number of battery cells of approximately 1.2 volts need to be connected in series. In the case where the rated voltage of the motor cannot be obtained even if a large number of battery cells are connected in series, the voltage of electric power discharged from the battery is increased by the DC-DC converter, and the electric power is supplied to the motor via the inverter.
Thus, in an electric circuit installed in a vehicle, it may be necessary to increase or decrease the voltage of electric power supplied to and from a battery. In such a case, a DC-DC converter is used. Japanese Patent Application Publication No. JP-A-H8-214592 discloses a drive apparatus for a motor in which the motor can be driven, regenerative braking can be performed, and charging of a battery can be performed using a simple configuration, and further the battery can be refreshed.
The drive apparatus for a motor includes a drive circuit; a chopper circuit; a direct current side reactor; and a control circuit. The drive circuit includes one or more arms formed by connecting two switching elements having a flywheel diode in series, and an input terminal thereof is connected to a battery, and an output terminal thereof is connected to the motor. Thus, the drive circuit controls the motor through energization by turning the switching elements on and off. The chopper circuit is connected to the drive circuit in parallel, and is formed by connecting two switching elements having a flywheel diode. The direct current side reactor is connected between a neutral point of the chopper circuit and the battery. The control circuit is provided for performing on-off control of the switching elements of the drive circuit and chopper circuit. The control circuit allows the chopper circuit to function as a chopper for increasing a voltage when electric power is supplied from the battery to the drive circuit, and to function as a chopper for decreasing a voltage when electric power is supplied from the drive circuit to the battery. Particularly, the control circuit performs control such that electric power the voltage thereof is equal to a reference voltage of the battery is supplied to the drive circuit when the output of the motor is low, and the chopper circuit functions as the chopper for increasing the voltage when the output of the motor is high.
In the drive apparatus for a motor disclosed in the Japanese Patent Application Publication No. JP-A-H8-214592, since the chopper circuit can function as a chopper for increasing the voltage when electric power is supplied from the battery to the motor via the drive circuit, a voltage higher than the battery voltage can be applied to the motor, and accordingly the motor can be driven at a speed higher than a steady-state speed. Also, since the chopper circuit can function as a chopper for decreasing the voltage when electric power is supplied from the drive circuit to the battery, the battery can be charged without causing damage to a circuit element even if the voltage of electric power generated by the motor or the voltage of an external electric power source is higher than the battery voltage in the case where electric power is generated by the motor during regenerative braking, or in the case where the battery is charged using an external electric power source.
Also, Japanese Patent Application Publication No. JP-A-H8-51800 discloses a control method in which a battery voltage is estimated such that control can continue to be performed even if an abnormality or the like occurs in a voltage sensor. The control method includes the steps of determining a primary voltage of a motor based on the battery voltage and a previous switching pattern; estimating a primary magnetic flux of the motor and a present value of motor torque based on the primary voltage of the motor and the primary electric current of the motor; determining a command value indicating the primary magnetic flux of the motor and a command value indicating the motor torque based on required output; deciding a switching pattern by comparing the estimated primary magnetic flux of the motor and the estimated present value of the motor torque and the determined command value indicating the primary magnetic flux of the motor and the determined command value indicating the motor torque; changing the primary electric current of the motor according to the decided switching pattern so as to control an induction motor. In the control method, the battery voltage is detected by the voltage sensor during normal operation, and the battery voltage is estimated based on a load state of the battery when an output of the voltage sensor is abnormal.
According to the control method, in the case where the voltage sensor outputs an abnormal value due to a failure of the voltage sensor for detecting the battery voltage, to a superimposition of noise onto the output of the sensor, or the like, the estimated value of the battery voltage is used. Accordingly, even when the voltage sensor outputs an abnormal value, control can continue to be performed according to a logic of high-speed direct torque control. Also, even when the estimated value of the battery voltage has an error, the control state does not become unstable as long as the error is not extremely large. Since the battery voltage is estimated based on the load state of the battery, the estimated value does not have a large error, and accordingly the control can be performed stably in a wide range.
As described above, in the drive apparatus for a motor disclosed in the Japanese Patent Application Publication No. JP-A-H8-214592, the chopper circuit is provided in parallel with the inverter circuit, the chopper circuit functions as a chopper for increasing the voltage when the output of the motor is high, and the chopper circuit functions as a chopper for decreasing the voltage when electric power is generated by the motor during regenerative braking. If an abnormality occurs in the voltage sensor when the voltage is adjusted in this manner, accurate voltage control cannot be performed, which may lead to deterioration of the battery and the like. Also, it is not possible to determine which one of the voltage sensor for detecting the increased voltage on an output side of the DC-DC converter and the voltage sensor for the battery is abnormal simply by comparing voltage values detected by these voltage sensors.
Also, according to the control method disclosed by the Japanese Patent Application Publication No. JP-A-H8-51800, a configuration including a DC-DC converter is not employed. An abnormality of the battery voltage sensor is determined simply based on a relation between the battery output and the battery voltage. If it is determined that the battery voltage sensor is abnormal, the induction motor is controlled using the estimated battery voltage. In other words, an abnormality of the battery voltage sensor is detected only when the relation between the battery output and the battery voltage deviates from the relation shown in a map.
Thus, in both cases, for example, when a voltage sensor for detecting the voltage that has not been increased is not provided on an input side of the DC-DC converter in order to suppress an increase of cost, and only the voltage sensor for detecting the increased voltage and the voltage sensor for the battery are provided, even if an abnormality occurs in one of the voltage sensors, it is not possible to detect an abnormality of the voltage sensor simply by comparing values detected by the voltage sensors.